Hydrogen is a potentially significant source of “clean” energy. That is, hydrogen reacts with oxygen to generate energy while producing water instead of the pollutants typically associated with the combustion of fossil fuels. However, the widespread use of hydrogen as an energy source requires its efficient, safe, and cost-effective storage.
Single-walled carbon nanotubes (SWNTs) may be used for, among other uses, reversibly storing hydrogen. SWNTs are well-known and generally comprise single layer tubes or cylinders in which a single layer of carbon is arranged in the form of a linear fullerene. Crude or low-purity SWNTs may be produced according to well-known processes such as arc discharge and chemical vapor deposition (CVD). SWNTs produced according to arc discharge processes tend to include graphite and/or graphite encapsulated metals, while SWNTs produced according to CVD processes tend to include other extraneous chemical compounds. Pure or easily purified SWNTs may also be produced according to other well-known processes, such as refined laser vaporization.
In any event, the hydrogen storage characteristics of SWNTs have been studied in the past. Hydrogen storage by low-purity, arc-generated SWNTs has been shown to occur at room temperature after first being heated to 973 degrees Kelvin (K) in a vacuum (<10−7 torr) to open and/or clean the tubes. However, when this approach was applied to laser-generated tubes, no significant hydrogen adsorption was observed. Other studies have also shown hydrogen uptake on pure SWNTs to be only negligible (about 0.1 weight percent (wt %)) at 100 atm and room temperature. Greater hydrogen storage densities of 5 to 10 wt % have been shown to occur at cryogenic temperatures (i.e., 80 degrees K) and high pressures (158 atmosphere (atm)). More recent studies have shown that large-diameter SWNTs (i.e., 1.85 nm) slowly adsorb about 4.2 wt % hydrogen at room temperature and about 100 atm over a period of about one hour. Although other studies have reported higher hydrogen adsorption, these studies have not been reproducible. See A. C. Dillon and M. J. Heben, Applied Physics A, 72 (2001) 133–142.
Consequently, a need remains for producing SWNTs that can be used for reversibly storing hydrogen. Additional advantages would be realized if the hydrogen adsorption and desorption process occurs without the need for high energy input, hence making the SWNTs amenable for use as hydrogen fuel cells. Further advantages would be realized if such a process were readily scalable, thereby allowing for the large scale, economical production of hydrogen storage systems.